The picture quality of CRT display is deteriorated when the CRT display device is magnetized by the effect of terrestrial magnetism. The CRT display is equipped with a degaussing circuit to degausse the magnetized CRT.
One example of the prior art is explained hereinafter referring to FIG. 1. Shown in FIG. 1 is the circuit diagram showing a conventional degaussing circuit. An AC power supply source 10 is connected to a positive temperature coefficient thermister (referred to as PTC, hereinafter) 12, a degaussing coil 14 and a relay switch 16 in series.
The relay switch 16 is kept closed for a fixed time period by a degaussing time period controller 18 which is interlocking with a manual switch 20 for starting the degaussing operation and applies AC current to the degaussing coil 14.
The PTC 12 is heated when a current is supplied thereto. Therefore, the resistance of the PTC 12 is increased.
When the resistance of the PTC 12 has increased, an AC current flowing through the degaussing coil 14 is gradually attenuated and thus, a degaussing effect is obtained.
In the prior art degaussing circuit as shown in FIG. 1, the PTC 12 is heated to increase its resistance, and thus gradually decrease AC current being supplied to the degaussing coil 14.
However, once the PTC 12 is heated, its temperature does not drop immediately and its resistance is also restored. Therefore, once the degaussation has been carried out, another operation of degaussation is difficult to effect immediately after the prior operation A required current for the degaussation cannot be supplied sufficiently to the degaussing coil 14 at that time. This is attributable to the fact that the PTC 12 once heated requires about 10 to 20 mins for dissipating its temperature before voltage application. During this time period, the resistance of the PTC 12 does not drop sufficiently for effective degaussing operations.
Therefore, if an orientation of the CRT display device is frequency changed, it is difficult to degausse the CRT display device immediately after every orientation change.
Further, as the degaussing coil 14 is connected to the AC power supply source, an electrostatic induction from the degaussation circuit to raster scanning circuits, such as a horizontal oscillation circuit and a vertical oscillation circuit, causes a picture fluctuation and a jitter on the screen.